A spun-filament fleece can be made in a device which includes a spinning nozzle system or spinneret, a cooling shaft, a stretching gap, a diffuser shaft, a continuously moving filament-receiving conveyor, and a device for feeding process air to the cooling shaft and for drawing exiting air through the mat-receiving conveyor. The cooling shaft has a shaft wall provided with a plurality of air orifices and the process air required for cooling is introduced through the air orifices to provide an air flow in the shaft that is at least partially drawable through the mat-forming conveyor.
A critical feature of a mat thus formed is the so-called deposited length which determines the thickness and appearance of the finished mat. More particularly it is noted that the filaments are generated and formed at a linear speed that is many times greater than the speed which the mat-receiving belt moves at. Thus these filaments will not extend in straight lines on the mat but instead will normally fall as coils or successions of loops. When the loops are elongated in the direction of displacement of the belt the deposited length is long, and when they are elongated transverse to this direction the deposited lenght is short. When the loops are transversely elongated, that is when the deposited length is short, the goods will be thicker and when they are elongated in the mat-travel direction the mat thus produced will be less thick.
According to the features of the known filament-spinning device the deposited lengths of filament loops which substantially determine the quality of the manufactured spun-filament fleece adjust themselves according to the flow rate of thermoplastic material which forms the endless filaments, in accordance with the flow rate of the process air, with the flow rate of exiting air, and with the geometry of the filament-spinning device and other parameters. If one keeps the described parameters constant, the deposited lengths of the filament loops cannot be readily changed particularly in regard to the spun-filament fleece thickness. If one changes the given parameters to adjust the deposited lengths, complex and not easily reproducible structures result.
In German patent document 2,260,135 filed 08 Dec. 1972 by Rudolf Brauer et al a system is described where the cooling shaft is constructed as a vertical venturi having relative to the direction of travel of the mat-receiving conveyor band a downstream wall and an upstream wall. The deposited length of the filaments in this case is controlled by changing the venturi shape by tilting the walls relative to one another. When the walls are made to diverge downward the result is a long deposited length and when they converge downward the deposited length is shorter.